This invention relates to a method used to compensate the tilting of electro-mechanical microwave tuners employed in on-wafer measurement operations by adding a balancing counter-weight on the mechanism supporting the tuner. Modern design of high power microwave amplifiers and oscillators, used in various telecommunication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is inadequate for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models only. Further on designing low noise amplifiers for communication systems requires exact knowledge of the noise parameters of microwave transistors to be used in the amplifiers; these can be obtained only by characterizing the microwave transistors by measuring their noise parameters.
A popular method for testing and characterizing such microwave components (transistors) in the small signal (low noise) and in the non-linear (high power) regions of operation is “source pull” and “load pull”. Source and Load pull are measurement techniques employing microwave tuners and other microwave test equipment. The microwave tuners are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor) is tested (FIGS. 1 and 2).
A typical load pull measurement system (FIG. 1) includes a signal source, an input coupler leading to an input power meter, a source tuner, a test fixture housing the DUT, a load tuner and an output power meter. The tuners and the overall test system are controlled by a control computer, which is connected to the tuners by control cables. Through digital commands from the computer the tuner's motors position the tuner's probes appropriately and create impedances, which allow characterizing the DUT (see reference 1).
Electro-mechanical slide screw tuners, compared to electronic tuners (see reference 2) or active load pull tuners (see reference 3), have a number of advantages like long-term stability, higher handling of microwave power, easier operation and lower cost. Such tuners (FIGS. 2 to 6) use adjustable mechanical obstacles (probes or slugs) inside the transmission media of the tuners; the transmission media are made using a slotted coaxial line or parallel plate airline (slabline), in order to reflect part of the power coming out of the DUT (device under test) and create this way adjustable microwave impedances presented to the DUT in order to perform the corresponding tests (see reference 4).
Electro-mechanical tuners comprise a solid housing (box), FIG. 3, a low loss transmission media for microwave energy (slotted transmission line or slabline) with an input or test port attached to a rigid bend-line, a moving carriage holding the microwave probe and electrical motors ensuring the horizontal and vertical movement of the carriage and the probe. The tuner is placed on a tuner positioner, which has bearings or gliders allowing a 3 axis geometrical movement (X-Y-Z). The carriage holding the probe comprises housing, a precision vertical gear and a vertical motor; this makes such carriages massive items with a typical weight on around 1 kg.
Control of the reflection factor phase is through horizontal movement of the carriage (see reference 4). Moving the massive carriage horizontally shifts the center of gravity of the whole assembly “tuner-bend-line-3 axis positioner” in such a way that the wafer probes touching the wafer chips may be crushed (FIGS. 2, 3). In order to cover a 360 degree circle of the reflection factor on the Smith Chart the free horizontal travel of the mobile carriage must be one half of a wavelength at the frequency of operation; for example at 1 GHz the free travel must be 15 cm, at 2 GHz 7.5 cm etc. A full noise or load pull characterization of transistors on wafer in this and lower frequency range requires the tuner mobile carriage to move horizontally over distances of this order of magnitude. It is during this operation that the wafer probe crushing problems appear, that this invention aims to solve.
The tilting problem of the tuner assembly is especially disturbing when using rigid bend-lines (FIGS. 2 to 4). Alternative configurations, like using semi rigid or flexible RF cables between the wafer probes and the tuners, would reduce this problem, but the cables represent an inferior solution from the RF point of view, because they require extra adapters and the core of the cables is filled with some dielectric material, like Teflon, which introduces higher insertion loss at microwave frequencies, thus reducing the effective tuning range of the tuners, i.e. the maximum reflection factor, that can be presented to the DUT.
The low loss bend-airlines create a rigid mechanical link between the tuners and the wafer micro-probes (FIG. 2). For on-wafer measurements using solid bend-lines a microwave tuner needs to be mounted on a 3-axis positioner (FIGS. 2 to 4) which manipulates the position of the probe together with the position of the tuner. Any mechanical movement inside the tuners like tilting of the whole assembly by the horizontal movement of the tuner carriages is translated on to the probes and creates a vertical probe movement of several dozens of micrometers. Considering that the contact pads on a wafer device under test (chip) have typical dimensions of 100 μm to 200 μm, it is obvious that even a small mechanical movement of the probes may easily damage the DUT contact pads and/or the wafer probes themselves. In order to avoid the vertical movement of the tuners and by consequence of the wafer probes the tuners must be ‘balanced’ whatever the actual position of the carriage is inside the tuner housing.
This is even more critical in the case of multi-carriage tuners. Such tuners can have 2, 3 or even more movable carriages, in which case the moving mass to contemplate can exceed 3 kg. The additional problem here is that the carriages do not move simultaneously and by the same amount in horizontal direction; instead each carriage moves independently of the others by a different amount and in the same or the opposite direction each time impedance is created. This creates the need of an intelligent independent compensation control allowing compensation of the overall total movement of the carriages and the resulting total shift of the center of gravity of the tuner system.
It is the purpose of this invention to solve the tilting problem using an intelligent automated mechanism of simultaneous tilting compensation (balancing) of the totality of the carriage movement.